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(Marcin) #1

52 Scientific American, April 2019 Illustration by Ben Gilliland


small things happening in the Planckian realm
might similarly influence phenomena accessible to
tabletop experiments. And although particle acceler-
ators cannot be upgraded by orders of magnitude—
we are unlikely to see accelerators with 1,000-kilo-
meter circumferences—the precision of tabletop
experiments may well improve by a few orders of
magnitude in the decades ahead.
Such gains might allow Aspelmeyer to test a key
assumption shared by all theories of quantum gravity:
that gravity itself should display some profoundly
strange quantum properties. “If that is really true,
there should be some consequences for phenomena
at an energy scale that is much much smaller [than
the high energies that correspond with the Planck
scale]”—that is, at roughly the scale we inhabit, Aspel-
meyer says. “The question is: Can we come up with
experiments that possibly test those consequence?”
What Aspelmeyer has in mind is an experiment
that would measure the gravitational attraction
between two spherical masses. Unlike Cavendish,

though, Aspelmeyer will not be weighing Earth, and
his milligram masses are orders of magnitude small-
er than Cavendish’s lead balls. He wants to test
whether gravity interacts at all with the quantum
properties of small masses. Specifically, he intends to
look at what kind of gravitational effects might be
generated by an object placed in a “Schrödinger’s
cat”-like state of being both here and there at once.
In the quantum world, particles have the uncanny
ability to be in two places simultaneously—a super-
position, as physicists call it. Scientists have observed
quantum superpositions many times in laboratories,
but they are delicate states. Interactions with any
nearby particles quickly cause objects in superposi-
tion to “collapse” into a single position. But while the
superpositions last, Aspelmeyer wonders what prop-
erties these particles have. Do they create their own
minuscule gravitational fields, for instance? “Imag-
ine you place an object in a superposition,” he says,
“and now you ask a question: How does it gravitate?
That is the question we want to answer.”

Laser
beam

< 1 μm

Falling diamond spheres in superposition

Spheres become entangled

Both masses in superposition

Test mass Superposition mass

Gold spheres

1 mm

Gravitational
field

Cantilever

Spring

Electromagnet

1 μm

PRELIMINARY EXPERIMENT #1
One experiment, proposed by physicist Markus Aspelmeyer, will ultimately
attempt to put a mass into a superposition state of being in two locations
simultaneously and then try to see if the gravitational field of the mass
splits into two as well. A preliminary version of this trial will develop the
technology to detect gravitational fields of smaller objects than ever
before—in this case, two tiny gold spheres. An electromagnet attached
to a spring will cause one ball to vibrate, and the other, at the end of a
cantilever, should oscillate in response to the changing gravitational pull.

ULTIMATE EXPERIMENT #1
Eventually the team will aim to put one of these spheres into a state
of superposition. If this ball’s gravitational field goes into superposition,
too, and exists in two places, then the other mass should feel the pull
of both fields and become entangled, entering superposition as well.

Quantum Gravity


Experiments


To understand whether gravity fits into quantum theory, physicists
are designing experiments to measure gravitational fields with
extreme precision to search for signs of quantum behavior. Such
behavior might include “superposition”—the ability of quantum
particles to occupy two places simultaneously—and “entangle­
ment”—a kind of connection between quantum objects where their
fates become intertwined. If researchers can find evidence of gravi­
tational fields displaying superposition or entanglement, they will
know that gravity has quantum properties.
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